The use of a wireless local-area network to detect the location of a device within the area covered by the network (the “WiFi triangulation method”) is advantageous because it permits devices to be located without the need to install additional hardware either in the environment or on the device being located. The WIFI triangulation method relies on characterizing each place by the received signal strength of the wireless access points that cover an area. A calibration phase is first done to record the values of the signal strength throughout the area in which devices are intended to be tracked. After the calibration phase the values of the access point signal strengths are used to find the most probable device location. In one implementation, the wireless triangulation location method compares measured wireless signal strength to a table of wireless signal strengths and known locations, finds the table entry with the closest signal strength to the measured signal strength and determines its location by reference to the found table entry.
However, the WiFi triangulation method suffers from the disadvantage that the signal strength not only changes as a function of location, but also changes due to many other factors, for example, changes in the environment (such as movement of people or large objects). These changes in the signal strength result in an inherent unreliability of the detected location; this problem may be referred to as the stability problem. In addition, the orientation of the mobile device can affect the signal strength received, meaning that the same location can result in several different levels of signal strength depending on the direction the device is facing; this problem may be referred to as the orientation problem.
One technique for improving reliability is to use a statistical process to find the most probable location based on a series of samples from the location algorithm, however this technique will perform badly if the user is moving, since there are actual changes in location as opposed to transient errors in location detection. Two other techniques, RADAR (P. Bahl and V. Padmanabhan, “RADAR: An In-Building RF-Based User Location and Tracking System,” Proc. IEEE Infocom 2000) and NIBBLE (P. Castro, P. Chiu, T. Kremenek, R. Muntz, “A Probabilistic Room Location Service for Wireless Networked Environments”, UbiComp 2001), implement systems for calculating the location of a mobile device based on signal strength information. In addition RADAR makes use of a fixed client to attempt to detect and compensate for overall variations in signal strength, however, this approach can only detect signal strength variations in the vicinity of the fixed client(s) which may not be representative of changes in other locations.
What is needed is a method of determining location using a WiFi network which minimizes or alleviates the stability problem. What is needed is a method of determining location using a WiFi network which minimizes or alleviates the orientation problem.